The present invention relates to a coating composition and its method of use for treating metal surfaces to apply a zinc phosphate conversion coating thereon. More particularly, the present invention relates to a zinc phosphate conversion coating composition which contains an accelerator system comprising chlorate anion and an aromatic nitro anion. The chlorate anion and aromatic nitro anion are employed within a weight ratio range of from less than 2:1 to about 1:10 to provide an improved, economical zinc phosphate coating composition which has low sludge and scale characteristics and which can be applied to a metal surface under low temperature conditions to provide an improved zinc phosphate conversion coating thereon.
The use of zinc phosphate conversion coatings as a surface pretreatment to prepare a metal surface for painting is well known. Conversion coating involves a non-electrolytic process which converts a purely metallic surface to an inorganic crystallinecoated surface having increased surface area and energy. The benefits offered by conversion coating include better paint adhesion over a longer period of time, increased corrosion resistance and generally improved paint durability.
Early processes for conversion coating required several hours of process time and provided only a small improvement in corrosion resistance. However, in recent years accelerators have been discovered which can be added to treatment solutions to reduce process times for conversion coating to a matter of seconds. Among accelerators which have been disclosed as useful in metal coating compositions are oxidizing agents such as are taught in U.S. Pat. Nos. 3,682,713 and 3,146,133; Canadian Pat. No. 698,386 and British Pat. No. 1,542,222, including agents such as chlorate and sodium m-nitrobenzenesulfonate which have even been used commercially. Heretofore, however, it was taught that chlorate and sodium m-nitrobenzenesulfonate should be used in relatively high weight ratios, e.g. from 2:1 to about 10:1.
As will be appreciated by those skilled in the art, corrosion resistance testing requires extended time periods even under accelerated conditions and accurate predictive testing procedures are difficult to define. As a result, coating compositions offering improved performance can be difficult to identify. Furthermore, although zinc phosphate coating compositions containing an aromatic nitro compound such as m-nitrobenzenesulfonate, and a chlorate can provide satisfactory performance in metal surface pretreatment, for some uses there remains a need for an improved zinc phosphate coating composition which economically offers performance improvements with regard to corrosion resistance and physical properties such as impact resistance, chip resistance, paint adhesion, and bend impact adhesion.
Wherefore, an improved zinc phosphate conversion coating composition has now been discovered wherein an accelerator system comprises chlorate anion and an aromatic nitro anion such as m-nitrobenzenesulfonate within a novel weight ratio range. A coating composition of the present invention provides an economical low sludge, low scale, low temperature zinc phosphate conversion coating having improved corrosion resistance and physical properties, especially when used under cathodic electrodeposition primer on steel, aluminum and galvanized surfaces.